In hybrid vehicles and electric vehicles, battery packs are used which feature lithium-ion or nickel-metal-hybrid technology and are composed of a large number of electrochemical cells which are connected in series. A control unit, also referred to as a battery management system, serves to monitor the battery and is intended to ensure a service life which is as long as possible, as well as monitoring the safety. For this purpose it is known to measure the voltage of each individual battery cell, the battery current and the battery temperature and to carry out estimations of states, for example of the state of charge (SOC) and/or the state of health (SOH).
In order to maximize the service life of the battery, it is necessary to know the current maximum state of function of the battery or the permissible operating range at all times. The permissible operating range of the electrochemical accumulators which are used can be represented, for example, by a multi-dimensional space which is formed by different state variables such as, for example, the temperature, the state of charge and the current strength. If the battery is operated at an operating point (for example temperature, state of charge and current strength) outside the permissible operating range, that is to say the performance limit of the battery is exceeded, the ageing of the battery can be unintentionally speeded up.
Electrochemical energy accumulators are optimized with respect to their power density and energy density for the respective use, that is to say the operating range is adapted in terms of size and position to the requirements or the field of use of the battery. It is therefore possible to assume that, in the peripheral areas of the operating range, the energy accumulator also actually runs up against its power limits, with the result that precise knowledge of the currently permissible operating range, that is to say of the maximum state of function, is extremely important. The electrochemical energy accumulators generally have a highly non-linear behavior at the boundaries of the operating range.
In order to determine the permissible operating range or the power values and energy values of an electrochemical accumulator as a function of the operating point, it is known to store data, which correspond to the operating range and which relate, for example, to the temperature, the state of charge and the current strength, in multi-dimensional tables. In this method, the measured data of the current operating point (voltage of battery cell, current and temperature of the battery) is compared with the data of the multi-dimensional tables and the permissible operating range of the relevant parameters is determined therefrom. However, this requires previous measurement of the corresponding properties of a structurally identical battery and requires low production variation. However, if significant production variation is present and if the electrochemical energy accumulator changes its properties in the course of time, this method becomes increasingly inaccurate.
In order to address the specified problem it is also known to examine individual battery cells or a representative selection of battery cells, by using a mathematical model not only with respect to their state of charge (SOC) but also with respect to ageing-specific parameters such as the state of health (SOH) and also to influence them by using a technical control structure, insofar as the permissible operating range does not permit the desired requirements of the battery to be met. A corresponding arrangement is illustrated schematically in FIG. 1. In said figure, the voltage VBatt of the battery cell 10, the battery current IBatt and the battery temperature TBatt are measured and an estimation of the state of charge SOC and the state of health SOH of the battery cell 10 is made by using a mathematical model. The processing of the measured values VBatt, IBatt, TBatt, and SOC and SOH are determined by using the means 14 for determining current operating parameters. The means 14 for determining current operating parameters can be formed, for example, by a microcontroller or some other data processing device, and said means 14 comprises part of the battery management system 12. The operating parameters SOC and SOH which are determined by the means 14 are transmitted to the means 16 for determining or predicting the permissible operating range of the battery cell 10. The means 16 for determining the permissible operating range can also be formed by a microcontroller or some other data processing device and said means also comprises part of the battery management system 12. The means 16 can use the determined cell-specific parameters SOC and SOH to make a model-based prediction of the power parameters (that is to say of the permissible operating conditions). Furthermore the permissible operating conditions which are determined by the means 16 can be compared with the actually expected requirements which are to be made of the battery and which are supplied by the means 18 for predicting requested operating parameters of the battery 10. On the basis of the requested operating properties and the permissible operating conditions it is possible to generate parameters for the actual operation of the battery within the permissible operating conditions, for example the discharge power P, the energy E which can be extracted and the state of function, also referred to as “state of function” (SOF). These parameters then control the operating point of the battery 10.
In this context, the means 14 and 16 preferably use the same mathematical model with identical parameters SOC and SOH. As a result, it is possible to dispense with previously measured and ascertained performance tables. The behavior of electrochemical energy accumulators can be considered in a simplified fashion as being linear in parts of the operating range and therefore be modeled by means of linear models. Linear mathematical models for determining operating parameters are, for example, the Shepherd model, the Unnewehr model or the method known from DE 602 03 171 T2.
In those parts of the operating range, generally in the peripheral zones, in which the behavior is non-linear, a model for describing the behavior can only be found with great effort. Such models are generally distinguished by a high level of complexity and a large number of parameters. Non-linear mathematical models for determining operating parameters are, for example, the Nernst model or the Plett model.